Microsoft has changed the name of First Release to Target Release.
This changes are coming from Roadmap ID: 24283
To understand more what rings are, see the image below.
To learn more please see here – https://support.office.com/en-us/article/Set-up-the-Standard-or-Targeted-release-options-in-Office-365-3B3ADFA4-1777-4FF0-B606-FB8732101F47?ui=en-US&rs=en-US&ad=US